Issue-2734 - Create static function for padding

Sources/NIOCore/ByteBuffer-core.swift

@@ -893,6 +893,7 @@ public struct ByteBuffer {
     /// - Returns: Bool indicating whether the buffer capacity has been shrunk to the desiredCapacity.
     @inlinable
     @discardableResult public mutating func shrinkBufferCapacity(to desiredCapacity: Int) -> Bool {
+        let desiredCapacity = ByteBuffer.addPaddingTo(desiredCapacity)
         guard desiredCapacity < capacity, desiredCapacity > readableBytes else {
             return false
         }
@@ -903,6 +904,14 @@ public struct ByteBuffer {
 
         return true
     }
+
+    /// Returns size of buffer capacity with optimal padding
+    /// - Parameter initialCapacity: Capacity that needs expansion with padding
+    /// - Returns: Capacity with calculated padding 
+    @inlinable
+    static func addPaddingTo(_ initialCapacity: Int) -> Int {
+        initialCapacity == 0 ? 0 : initialCapacity.nextPowerOf2()
+    }
 
     /// The reader index or the number of bytes previously read from this `ByteBuffer`. `readerIndex` is `0` for a
     /// newly allocated `ByteBuffer`.

Sources/NIOCore/Codec.swift

@@ -797,7 +797,7 @@ public final class MessageToByteHandler<Encoder: MessageToByteEncoder>: ChannelO
 
     public init(_ encoder: Encoder, desiredBufferCapacity: Int) {
         self.encoder = encoder
-        self.desiredBufferCapacity = desiredBufferCapacity.nextPowerOf2()
+        self.desiredBufferCapacity = desiredBufferCapacity
     }
 
     public init(_ encoder: Encoder) {

Tests/NIOCoreTests/ByteBufferTest.swift

@@ -1891,6 +1891,11 @@ class ByteBufferTest: XCTestCase {
         XCTAssertTrue(buffer.shrinkBufferCapacity(to: desiredCapacity))
         XCTAssertEqual(buffer.capacity, 1024)
     }
+
+    func testExpansionOfCapacityWithPadding() throws {
+        XCTAssertEqual(ByteBuffer.addPaddingTo(12), 16)
+        XCTAssertEqual(ByteBuffer.addPaddingTo(0), 0)
+    }
 
     func testDumpBytesFormat() throws {
         self.buf.clear()